1. Field of the Invention
The present invention relates to a method of crystallizing silicon, and more particularly, to a method of crystallizing silicon that reduces an inner grain defects.
2. Description of the Related Art
Currently, as the demand for providing information increases, various types of flat panel display devices are being developed that have slim profiles, are lightweight, and have low power consumption. Among the various types of flat panel display devices, liquid crystal display (LCD) devices are being used to provide superior color reproduction.
In general, the LCD devices include two substrates facing each other with both the two substrates having electrodes formed on the facing surfaces of the two substrates, and liquid crystal material injected into a space defined between the two substrates. Accordingly, the LCD display devices produce images by manipulating liquid crystal molecules of the liquid crystal material due to an electric field generated by voltages applied to the electrodes in order to vary light transmissibility through the liquid crystal material.
The lower substrate of the LCD device includes thin film transistor (TFTs) each provided with an active layer that is commonly formed of amorphous silicon (a-Si:H). Since the amorphous silicon can be deposited as a thin film onto a low melting point glass substrate at relatively low temperatures, it is commonly used for forming switching devices of LCD panels. However, amorphous silicon thin films deteriorate electrical characteristics and reliability of the switching devices of the LCD panels. Accordingly, amorphous silicon thin films are difficult to use in large-sized LCD screens.
For commercial application of LCD devices used in laptop computers and large-sized wall-mountable TVs, a pixel driving device having improved electric characteristics, such as high electric field effect mobility (30 cm2/Vs), radio frequency operational characteristics, and low leakage current is required. To improve these electric characteristics, high quality polycrystalline silicon (i.e., polysilicon) is required. The electric characteristics of polysilicon thin films are dependent upon grain size. For example, increasing the grain size, increases electric field effect mobility.
Various methods for crystallizing silicon into single crystalline silicon have been disclosed in PCT Publication No. WO 97/45827 and Korean Patent Laid Open Publication No. 2001-004129, which teach sequential lateral solidification (SLS) techniques for making massive single crystalline silicon structures by inducing lateral growth of silicon crystals using lasers as energy sources. The SLS technique has been developed based upon the fact that silicon grains are grown along a direction normal to a boundary surface between liquid silicon and solid silicon. The SLS technique crystallizes amorphous silicon thin film by letting the silicon grains laterally grow to predetermined lengths by appropriately adjusting laser energy intensities and laser beam projection ranges.
FIG. 1 is a schematic view of an apparatus for crystallizing silicon using an SLS technique according to the related art. In FIG. 1, an SLS apparatus 100 comprises a laser generator 111 for generating a laser beam 112, a convergence lens 113 for converging the laser beam 112 irradiated from the laser generator 111, a mask 114 for dividing the laser beam into a plurality of sections and projecting the divided sections onto a panel 116, and a scale lens 115 for reducing the laser beam 112 passing through the mask 114 to a predetermined scale.
The laser generator 111 irradiates a raw laser beam 112, the irradiated raw laser beam 112 is adjusted in its intensity while passing through an attenuator (not shown), and is then transmitted to the mask 114 through the convergence lens 113. The panel 116 is deposited with amorphous silicon, which is disposed on an X-Y stage 117, corresponding to the mask 114. At this point, to crystallize an entire area of the substrate 116, a method for gradually enlarging the crystallized area by minutely moving the X-Y stage 117 is used. Accordingly, the mask 114 is divided into laser beam transmission regions 114a, which allow for transmission of the laser beam 112, and laser beam interception regions 114b, which absorb the laser beam 112. A distance between the transmission regions 114a (i.e., a width of each interception region 114) determines a lateral grown length of the grains.
A method for crystallizing the silicon using the SLS apparatus includes forming crystalline silicon using amorphous silicon deposited onto a buffer/insulating layer (not shown), wherein the buffer layer is formed on the substrate 116. The amorphous silicon layer is deposited onto the substrate 116 by, for example, a chemical vapor deposition (CVD) process, wherein large amounts of hydrogen can be contained within the amorphous silicon layer. Since the hydrogen contained in the amorphous silicon layer is generally outdiffused from the thin film by heat, the amorphous silicon layer should undergo a dehydrogenization process through heat treatment. That is, when the hydrogen is not removed in advance from the amorphous silicon layer, the crystallized layer may become exfoliated due to rapid volume expansion of hydrogen gas contained within the amorphous silicon layer during the crystallization process.
In addition, the crystallization process using the laser cannot simultaneously crystallize the entire area of the surface. For example, since a width of the laser beam 112 and a size of the mask 114 are limited, the mask 114 must be realigned many times and the crystallization process must be repeated whenever the mask 114 is realigned in order to crystallize a large-sized screen panel. Accordingly, an area that is crystallized using the area of the mask 114 is referred to as a unit block, wherein the crystallization of the unit block should be realized by repeatedly irradiating the laser beam.
FIG. 2 is a silicon crystallization diagram according to the related art. In FIG. 2, a first regime on the diagram represents a partial melting region in which only a surface portion of silicon is melted to form small-sized grains. A second regime represents a nearly complete melting region in which larger grains than the first regime can be formed. However, it is difficult to obtain uniform sized grains. A third regime represents a completely melting region in the amorphous silicon is completely melted and fine grains are formed by homogeneous nucleation.
According to the related art, crystallizing silicon using the SLS technique is performed within the completely melting region (i.e., third regime) in which amorphous silicon is completely melted by laser crystallization.
FIG. 3 is a schematic plan view of a mask for crystallizing silicon using the SLS technique according to the related art. In FIG. 3, a mask 114 comprises patterned transmission and interception regions 114a and 114b, where each of the transmission regions 114a is defined by a longitudinal slot extending along a first direction. A width “d” of the transmission region 114a is designed to be less than twice as long as a maximum length of a grain grown by a first laser irradiated process. When the first laser beam is irradiated through the mask 114, grains are laterally grown from both boundaries of the molten amorphous silicon within the melting regions of the amorphous silicon layer, which correspond to the transmission regions 114a of the mask 114 until boundaries of the laterally grown grains collide with each other at a middle line of a liquid phase.
During the crystallization process, the beam pattern passes through the mask 114 and is reduced by a scale lens 115 (in FIG. 1) that moves along an X-axis direction. Accordingly, the crystallization process proceeds while the laser pattern moves in units of hundreds of μm (i.e., a length of a pattern reduced by the scale lens 115).
FIGS. 4A to 4D are cross sectional views of the SLS technique according to the related art. In FIGS. 4A to 4D, a 2-shot SLS polysilicon crystallization method is described, wherein three transmission patterns (regions) are defined on the mask. In the 2-shot polysilicon crystallization method, regions of the amorphous silicon layer that correspond to the transmission regions are crystallized by irradiating the laser beam twice. In addition, the crystallization process is consecutively carried out along a lengthwise direction of a substrate. When the crystallization is completed along the lengthwise direction of the substrate, the laser pattern minutely moves along a widthwise direction, and then moves along the lengthwise direction to proceed with the crystallization, thereby completing the crystallization process for the desired region.
In FIG. 4A, the mask 114 (in FIG. 3) is first located corresponding to the substrate, and the first laser beam is irradiated to proceed with the crystallization process for the amorphous silicon layer deposited onto the substrate. The irradiated laser beam is divided into a plurality of sections while passing through the plurality of slits 114a (in FIG. 3) formed in the mask 114. Accordingly, regions of the amorphous silicon layer, which correspond to the slits 114a, are liquefied by the divided sections of the first laser beam. In this case, laser energy intensity is set to be within a completely melting region in which the amorphous silicon layer can become completely molten.
When the laser beam irradiation is completed, silicon grains are laterally grown at a boundary between the solid amorphous silicon region and the liquefied amorphous silicon region. A width of the beam pattern passing through the mask is set to be less than twice as long as a length of the grown grains, and the crystallized regions correspond to the transmission regions 114a (in FIG. 2) of the mask. Accordingly, each of the crystallized regions A1, A2, and A3 has a length identical to that of each of the transmission regions 114a. In addition, regions of the amorphous silicon layer, which correspond to the interception regions 114b (in FIG. 2) of the mask, remain as amorphous silicon regions 167.
Accordingly, the grains 166a and 166b are laterally grown from the boundaries between the liquefied silicon and the solid silicon within the crystallized regions A1, A2, and A3, thereby defining a grain boundary, as shown in FIGS. 4A to 4C.
Next, the crystallization process is consecutively carried out along an X-axis direction while the stage upon which the substrate is disposed moves by units of hundreds of μm that are as long as a length of the mask pattern (beam pattern). In FIG. 4B, when the first crystallization along the X-axis direction (i.e., horizontal direction) is completed, the mask 114 on the X-Y stage 117 (in FIG. 1) is minutely moved along a Y-axis direction (i.e., vertical direction). Next, a second laser irradiation is initiated from a point where the first crystallization is finished along the X-axis direction. Accordingly, the grains of the crystallized silicon formed by the first laser irradiation are further consecutively grown by the second laser irradiation. For example, the grains are further grown to have a length that is one-half as long as a distance “k” from the grain boundary 166c of the crystallized region A1 to the grain boundary of the adjacent crystallized region A2.
In FIG. 4C, a polysilicon thin film formed of the grains 168a and 168b having a predetermined length can be formed. Accordingly, within newly crystallized regions B1 and B2, grains 168a and 168b are vertically grown from the boundaries between the liquefied silicon and the solid silicon. The grains 168a and 168b are further grown until they contact each other, thereby defining a new grain boundary 168c, as shown in FIG. 4D.
Although the method of crystallizing amorphous silicon using the SLS technique is performed within the complete melting regime corresponding to a region where amorphous silicon is completely melted, many defects 169 (in FIG. 4D) are contained within large-sized grains. These defects 169 (in FIG. 4D) are repaired by a second Excimer laser annealing (ELA) process within the near complete melting region having a lower energy than the completely melting region. However, the method using first a laser with a high energy and then using a laser with a low energy as an additive process increases total processing time, thereby decreasing process yield.